This invention relates to a field effect transistor (hereinafter referred to as an "FET") made of silicon carbide (hereinafter referred to as "SiC") that operates under severe conditions, including elevated temperatures and elevated levels of ionizing radiation. In particular, this invention relates to SiC power semiconductor devices such as a vertical FET and a MESFET.
Silicon carbide has a wide band gap and is chemically stable. Semiconductor devices made from SiC operate successfully at higher temperatures and higher levels of ionizing radiation than silicon devices. The maximum operating temperature of conventional silicon devices is around 150.degree. C. In contrast, some prototype SiC element devices such as p-n junction diodes and MOSFET's (FET's having a gate with a metal-oxide film-semiconductor structure) have been fabricated and operated successfully above 400.degree. C. SiC devices operable at such high temperatures are quite useful for robots and computers in environments too severe for humans, such as near nuclear reactors or beyond the earth's atmosphere.
Conventional silicon devices require cooling to prevent overheating during operation. The cooling typically involves bulky radiator fins which complicate silicon device structure. In contrast, using SiC instead of Si in an FET permits simpler and smaller semiconductor packages. Use of SiC reduces weight and improves fuel consumption in those instances where semiconductor devices make up a large part of an apparatus, such as in an automobile. Thus, SiC devices are useful where environmental considerations are important.
Certain technical problems exist in manufacturing SiC power devices. The most difficult problem is obtaining deep impurity diffusion. The diffusion coefficients for impurities in SiC are much smaller than those for silicon. Therefore, high temperature heat treatment at 1500.degree. C. or more is needed in order to achieve sufficiently deep diffusion in SiC. However, materials which tolerate such high temperatures are quite limited. It is also difficult to operate an electric furnace at that high a temperature with sufficient stability. Moreover, air reacts vigorously with the specimen at high temperatures, significantly affecting the device surface. Therefore, inventing a SiC device structure that eliminates the above difficulties is desirable.
Referring to FIG. 8, a vertical MOSFET according to the prior art is shown. The vertical MOSFET is an important device structure for applying SiC to power semiconductor devices. Since the vertical MOSFET is voltage driven, its driving circuits are simple. The vertical MOSFET is a monopolar device which switches at high speed. Although deep diffusion is difficult on SiC, epitaxial growth is relatively easy. Therefore, a trench MOSFET is popular for SiC devices.
A p-type base layer 83 is more easily formed by epitaxial growth than by thermal diffusion. An n-type drift layer 82, more lightly doped than an n.sup.+ substrate 81, is epitaxially grown on the n.sup.+ substrate 81. The p-type base layer 83 is epitaxially grown on the n-type drift layer 82. The no substrate 81, the drift layer 82, and the p-type base layer 83 constitute a SiC base plate. An n-type source region 84, which is heavily doped, is selectively formed in a surface portion of the SiC base plate. A trench 85 is formed from a part of the surface of the n-type source region 84 down into the n-type drift layer 82. A gate insulation film 86 covers the trench 85, with a gate electrode 87 fixed to the gate insulation film 86. A source electrode 88 contacts both the n-type source region 84 and an exposed surface portion of the p-type base layer 83. The n.sup.+ substrate 81 has a drain electrode 89 disposed on its back surface.
During operation of the vertical MOSFET, a positive voltage that exceeds a threshold value is applied to the gate electrode 87. At the same time, a voltage is applied between the drain electrode 89 and the source electrode 88, creating an inversion layer in the surface portion of the p-type base layer 83 beside the gate electrode 87. An electron current flows from the source electrode 88 to the drain electrode 89 through this inversion layer. The gate insulation film 86, formed from silicon dioxide, is made by thermally oxidizing SiC.
A hole current flowing through the p-type base layer 83 during switching of the SiC vertical MOSFET causes a voltage drop across the resistance of the p-type base layer 83. This voltage drop forward biases a p-n junction between the n-type source region 84 and the p-type base layer 83, and drives a resulting parasitic npn transistor to cause the device to break down.
The usual power devices are required to withstand an avalanche current of a certain value. However, the capability of the conventional SiC vertical MOSFET to withstand avalanche, as defined by the insulation breakdown of the gate insulation film, is small since the breakdown starts from the trench of the gate portion.
Referring to FIG. 9, a MESFET (metal semiconductor field effect transistor) according to the prior art is shown. The MESFET is an important application of SiC to power semiconductor devices. (See, e.g., J. W. Parmer et al., "Diamond, Silicon Carbide, and Nitride Wide Band Gap Semiconductors", Proceedings of Materials Research Society, (1994).)
A p-type epitaxial layer 95 is formed on an n.sup.+ substrate 91. An n.sup.- base layer 93 is grown on the p-type epitaxial layer 95. A heavily doped n.sup.+ layer is formed on the n.sup.- base layer 93. The heavily doped n.sup.+ layer is then selectively removed leaving a heavily doped n.sup.+ source layer 94 and an n.sup.+ drain region 90. A Schottky electrode 97 is disposed on an exposed surface portion of the n.sup.- base layer 93. A nickel film is deposited on the heavily doped n.sup.+ source layer 94 and the n.sup.+ drain region 90 by sputtering, forming a source electrode 98 and a drain electrode 99.
During operation of the MESFET, a current flows when a voltage is applied between the drain electrode 99 and the source electrode 98. A depletion layer is expanded into the n.sup.- base layer 93 beneath the Schottky electrode 97 by applying a negative voltage exceeding a certain threshold value to the Schottky electrode 97. The current is interrupted when the depletion layer reaches the p-type epitaxial layer 95.
Since the current carrying region of the conventional MESFET has to be narrow enough so that a depletion layer sufficiently expands therein in the OFF state of the MESFET, the ON-resistance of the current carrying region is too high to permit the high current required from power devices.